The invention generally relates to fuel cell technology. More particularly, some embodiments are directed to bond layers used to join various structures in solid oxide fuel cells.
Solid oxide fuel cells (SOFC's) are promising devices for producing electrical energy from a fuel, with high efficiency and low emissions. Solid oxide fuel cells rely on electrodes, electrolytes, and various other features and structures which are capable of withstanding operation at high temperatures (e.g., about 800° C.). In view of considerable material- and fabrication costs, solid oxide fuel cells require relatively large cell structures; as well as high power densities and fuel utilization levels, to make the technology economically feasible. Multiple factors influence the overall performance of solid oxide fuel cells and fuel cell stacks. These include electrolyte ohmic resistance, electrode polarization, mass transport limits, and the contact resistance at various interfaces in the devices.
As is known in the art, a single fuel cell is based on a sequential structure which includes an anode, a cathode, and an intervening electrolyte. Commercial solid oxide fuel cell structures usually consist of many of these cells stacked together—sometimes hundreds of cells, which cumulatively provide enough voltage to make the device commercially feasible. The cells are typically joined together by interconnects, which are usually in the form of metallic or ceramic layers. The interconnects provide electrical contact, current distribution, and structural integrity between individual cells.
In a typical cathode-electrolyte-anode stack arrangement (viewed vertically for the sake of discussion), one interconnect layer is attached to an upper surface of a cathode layer, for connection to the anode layer of an adjacent cell or “module”. Another interconnect layer is attached to the lower surface of the anode, for connection to the cathode layer of another adjacent cell. In view of the fact that interconnects may be exposed to both the oxidizing and reducing side of the cells at high temperatures, they must be extremely stable. For this reason, ceramics have typically been the best choice for long term use, as compared to metals.
Solid interconnects may not always allow optimum performance in a solid oxide fuel cell. As an example, the solid interconnect layer may become warped or otherwise deformed, due to high temperatures, temperature cycling, coefficient-of-thermal-expansion (CTE) differences, and the like. In these cases, the interconnect may not firmly and completely contact the adjacent electrode, leading to variable or insufficient electrical contact through the cell.
One means of addressing the drawbacks of solid interconnects involves the use of a bond paste or bond layer, as described, for example, in U.S. Pat. No. 6,949,307. The bond layer is typically a porous, electronic-conducting ceramic layer. It can be formed on or applied to the surface of the electrode (e.g., a cathode); to the facing surface of the interconnect, or in both locations. The bond layer improves electrical contact between the interconnect and the adjacent electrode. The bond layer can also improve the dimensional tolerance between the interconnect and the electrode, and thereby enhance the physical integrity of the solid oxide fuel cell stack.
As in the case of the other layers and structures in the solid oxide fuel cell, the characteristics of the cathode bond layer affect cell performance. As alluded to above, the bond layer permits the transport of oxygen from the air source for the fuel cell, to the cathode-electrolyte interface. The bond layer also functions to transport electrons from the outer circuit to the cathode-electrolyte interface.
Thus, in order to carry out its intended function, the cathode bond layer must be formed of a material which provides a desired level of oxygen mass transport and electrical conductivity under fuel cell operating conditions. The bond layer must also be chemically and physically stable under those conditions, and must not contain constituents which would adversely react with other cell components.
It may also be desirable for the cathode bond layer to exhibit a relatively high degree of mechanical strength. In most instances, the bond layer should also have thermal expansion characteristics which are compatible with those of the other fuel cell components, e.g., the cathode and adjacent interconnect. In view of the continuing interest and developments in solid oxide fuel cells, cathode bond layers which exhibit improvements in some or all of these properties would be welcome in the art.